Phase transitions and relaxor behavior of lead-free ferroelectric bismuth potassium titanate

The phase transition behavior of relaxor ferroelectric Bi1/2K1/2TiO3 (BKT) is investigated in ceramic form based on synchrotron radiation x-ray diffraction (SR-XRD) measurements and dielectric measurements as a function of temperature. The BKT ceramics show a spontaneous phase transition from the non-ergodic relaxor (NER) phase to the ferroelectric phase characterized by an abrupt change in relative permittivity around 310 °C. Analyses of the temperature dependence of dielectric properties reveal that, upon cooling from the high-temperature paraelectric phase, a phase transition to the ergodic relaxor phase associated with the formation of polar nanoregions (PNRs) occurs at the Burns temperature (TB) of 584 °C, and the PNRs are frozen at the freezing temperature (Tf) of 364 °C. SR-XRD measurements for the BKT powders show that the crystal structure transforms from the cubic (Pm3¯m) to the tetragonal (P4mm) in the temperature range of 327–347 °C, which is slightly higher than the temperature of the spontaneous relaxor-to-ferroelectric phase transition. These results indicate that the structural transformation from the cubic NER state to the tetragonal NER state may trigger the spontaneous relaxor-to-ferroelectric phase transition. Based on these experimental observations, a temperature phase diagram of BKT is established in terms of average crystal structure and polar order.